I. Field of the Invention
The present invention relates generally to servo systems comprising hydro-mechanically driven actuators and, more particularly, to enhancing high frequency shudder stability of such systems with particular reference to their utilization in vehicular power steering systems.
II. Description of the Prior Art
The widest use of servo systems comprising hydro-mechanically driven actuators is found in the vehicular power steering art where high frequency shudder has been a major problem. High frequency shudder is the result of inherent dynamic instability found in many vehicular power steering systems. It is known in the art to reduce high frequency shudder by a variety of techniques including utilizing elastomeric bushings within tie-rod linkage assemblies, reducing steering control valve gain, modifying fluid supply line assemblies, and adding supplemental motion control dampers. Among these techniques, damping provided by supplemental motion control dampers can effectively modify servo feedback characteristics of vehicular power steering systems such that their operational stability is significantly enhanced. However, such supplemental motion control dampers result in increased steering efforts with respect to steering wheel rotation and generally take the form of gas-filled shock absorbers which are relatively expensive and difficult to mount. Thus, they are usually added to vehicular power steering systems only as a last resort.
The other three stabilizing methods are more commonly utilized. Their functions differ in fundamental ways. Elastomeric bushings in tie-rod linkage assemblies tend to eliminate harshness and excessive sensitivity in "down-the-road" and handling phases of vehicle maneuvering by increasing compliance and introducing series damping between steering gear and dirigible wheel portions of the host power steering system. The elastomeric bushings also delay onset of shudder until higher values of steering force are encountered such as during parking on dry pavement or brushed concrete with the host vehicle stationary. However, when steering loads reach such high levels, the elastomeric bushings are usually so heavily loaded that they loose most of their compliance and damping qualities, and they are substantially ineffective in eliminating shudder.
On the other hand, reducing control valve gain is thought to enhance system stability in a manner similar to that provided by motion control dampers. However, as will be fully explained below, this procedure is only marginally effective in modifying servo feedback characteristics and reducing shudder. Because of this, associated fluid supply lines are often tinkered in an attempt to negate system disturbances generated by the host power system's power steering pump, as is also described below.
Stability problems associated with servo systems are discussed by DiStefano, Stubberud, and Williams in Schaum's Outline of Theory and Problems of Feedback and Control Systems published by the McGraw-Hill Book Company. As discussed in that book, servo systems can oscillate via self excitation at any frequency whereat unity gain open-loop feedback coincides with an odd multiple of 180 degree phase shift. However, shudder instability is rarely self excited because exact combinations of unity gain open-loop feedback coupled with odd multiples of 180 degree phase shift are seldom encountered. Rather, such servo systems are characterized by near confluence of these conditions over an unusually wide frequency span. Thus, shudder instability is usually encountered as an amplification of periodic variations in hydraulic flow provided by an engine driven power steering pump. Typically an engine operating at low speeds (especially when the engine is operated in a loaded condition such as being in gear with the air conditioner on) has significant variations in its speed. Since the pump is driven by the engine, it has substantially identical variations in both speed and output flow rate. The primary cause of the variation of engine rotational speed is the individual power pulses determined by the firing frequency of the engine's cylinders. A six cylinder engine has three power strokes per revolution and when operated at idle speed in a manner such as that described above, may have a firing frequency of about 33 Hz with an associated rotational speed ripple of approximately 10% from peak-to-peak.
When operated in a parking mode as described above, the host vehicle's steering gear is under maximum load. At maximum load its control valve is internally deflected such that it significantly restricts pump output flow in order to concomitantly develop maximum values of pump output pressure. When the control valve is so deflected it couples the pump output pressure to a power piston member of the steering gear in a substantially direct manner. Since the pump output pressure is generated by a combination of closing variable flow control orifices, its value is primarily governed by the general orifice flow equation EQU Q=C.sub.d A(2P/.rho.).sup.0.5
where Q is flow through the control valve, the product C.sub.d A is net effective valve orifice area, .rho. is fluid density and P is the pump output pressure. As a result, the pump output pressure is proportional to the square of pump output flow and is thus subject to an approximate 20% peak-to-peak variation at about 33 Hz. Because of this, supply line assembly materials and lengths are often altered in an attempt to achieve line resonances with an approximate dynamic pressure null at the control valve. Sometimes it is additionally necessary to provide a flow restricting device in the return line of the supply line assembly. Tinkering with the supply line assembly to achieve high frequency stability is generally regarded as being marginally acceptable at best because the underlying stability problem still exists in unaltered form. Thus, it is present in all vehicles so equipped and can reappear dramatically in selected vehicles that differ from norm. The differences between supposedly identical vehicles are quite undefined and can occur in the form of such things as missed structure welds, loose fasteners, unusual tolerance buildups and the like.